The present application relates to a secondary battery equipped with an electrolytic solution as well as a positive electrode and a negative electrode and to a battery pack, an electric vehicle, an electric power storage system, a power tool, and an electronic appliance, each using the secondary battery.
In recent years, a variety of electronic appliances represented by a mobile phone, a personal digital assistant (PDA), or the like have widely spread, and it is strongly demanded to realize even more downsizing, weight reduction, and long service life thereof. Following this, the development of, as an electric power source, a battery, in particular, a secondary battery which is small-sized and lightweight and from which a high energy density is obtainable, is being advanced. Recently, as for this secondary battery, applications to a variety of uses represented by a battery pack to be detachably mounted on electronic appliances, an electric vehicle such as electric cars, an electric power storage system such as household electric power servers, and a power tool such as power drills are also investigated.
As the secondary battery, those utilizing a variety of charge and discharge principles are widely proposed. Above all, a secondary battery utilizing intercalation and deintercalation of an electrode reactant is regarded as promising. This is because a higher energy density than that of a lead battery or a nickel-cadmium battery or the like is obtainable.
The secondary battery is equipped with an electrolytic solution as well as a positive electrode and a negative electrode. A positive electrode active material layer of the positive electrode contains, in addition to a positive electrode active material capable of intercalating and deintercalating an electrode reactant, other materials such as a binder. A negative electrode active material layer of the negative electrode contains, in addition to a negative electrode active material capable of intercalating and deintercalating an electrode reactant, other materials such as a binder. The electrolytic solution contains a solvent and an electrolyte salt.
In general, a lithium transition metal complex oxide such as LiCoO2 is used as the positive electrode active material, and also, a carbon material such as graphite is used as the negative electrode active material. In addition, a mixed solvent of a cyclic carbonate that is a high-dielectric solvent and a chain carbonate that is a low-dielectric solvent, or the like is used as a solvent of the electrolytic solution. This mixed solvent is not only a polar solvent capable of sufficiently dissolving or ionically dissociating the electrolyte salt such as LiPF6 but an aprotic solvent capable of rapidly transmitting an ion at the time of charge and discharge.
The electrolytic solution using this mixed solvent tends to exhibit high viscosity and surface tension to be caused due to an interaction by polarity. In that case, if a binder such as polyvinylidene fluoride is contained in the positive electrode active material layer or negative electrode active material layer, the affinity of the electrolytic solution becomes low, and therefore, a penetration rate of the electrolytic solution into the positive electrode active material layer or negative electrode active material layer becomes slow. According to this, a time is required for a penetration work of the electrolytic solution in a manufacturing step of a secondary battery, and therefore, a lowering of the production efficiency is incurred. In addition, the positive electrode or negative electrode into which the electrolytic solution is not sufficiently penetrated is substantially difficult to participate in a charge and discharge reaction, and therefore, a lowering of the battery capacity is also incurred.
Then, in order to improve the penetration properties of the electrolytic solution into the positive electrode or negative electrode, it is proposed to allow a surfactant having a polar segment and a non-polar segment to be contained in the positive electrode or negative electrode or the electrolytic solution (see, for example, Patent Documents 1 (JP-A-1995-263027) and 2 (JP-T-2009-526349)). In that case, for the purpose of feeding an electrode reactant at the time of initial charge and discharge to reduce the irreversible capacity, it is also proposed to use a lithium alkyl carbonate having a carbon number of the alkyl group thereof of from 1 to 4 (see, for example, Patent Document 3 (Japanese Patent No. 4270904)).